During the course of a typical communications session between users in a communications system, a good deal of the information communicated may be non-essential, consisting of background noise or other information which is of little or no use to the end users. For example, in an audio application such as radiotelephony, each user may be "silent," e.g., not communicating voice or other desired audio information, more than 50% of the time during a communications session. Communication of silence or other content-less background audio over the channel generally represents an inefficient use of system resources, e.g., bandwidth, channel capacity, and power. Thus, it may be desirable to discontinue transmission from a station over the channel when the audio signal at the station carries no voice information, in order to conserve energy and to allow other users to use the channel during idle periods.
For this reason, discontinuous transmission (DTX) is often used in communications systems. In conventional cellular radiotelephone systems, for example, DTX may be implemented by communicating an explicit message containing information as to transitions of the state of a transmittingstation, e.g., active-to-idle, idle-to active, to a receiving station in order to control reception. This status message may be communicated on a dedicated status channel or as a message on a traffic channel. For example, a "silence descriptor," i.e., an explicit message indicating lack of voice activity, may be transmitted from the transmitting station to indicate voice inactivity, as described in U.S. Pat. No. 5,475,712 to Sasaki.
DTX can help improve spectral efficiency by freeing channels for use by other users during idle periods. In addition, discontinuous transmission can reduce co-channel interference, by reducing the probability of collisions between channels. Reduced co-channel interference can in turn allow higher channel density, as the likelihood of unacceptable interference between channels is reduced.
DTX offers additional benefits for power-sensitive applications such as mobile satellite radiotelephone applications. In addition to having finite bandwidth and channel capacity, the energy available for transmission of communications signals from the satellite or the mobile unit tends to be limited. Thus, it generally is desirable to avoid wasting satellite energy on the communication of non-essential audio information. By employing DTX, a satellite-based communications system can conserve satellite and mobile power by preventing the transmission of communications signals corresponding to non-essential audio information.
Unfortunately, however, conventional DTX techniques may still consume a significant amount of system resources. Dedicated signalling channels can occupy bandwidth which might otherwise be available for traffic, and consume energy in the communication of status signals. Similarly, the use of silence descriptors may decrease available channel capacity and increase power consumption in the transmission of the silence descriptor.